As known in the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In groups of drying cylinder employing twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one of which presses the web from above and the other of which presses the web from below, against heated cylinder faces of the drying cylinders. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible to fluttering, which may cause web breaks, in particular when the web is still relatively moist and therefore has a low strength. For this reason, in recent years, ever increasing use has been made of drying cylinder groups employing single-wire draw, in which each group of drying cylinders includes only one drying wire, on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas the web remains at the side of the outside curve on the reversing cylinders or rolls arranged between the drying cylinders. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
In so-called normal groups with single-wire draw, known in the prior art, the heated drying cylinders are placed in an upper row and the reversing cylinders are placed in a lower row below the upper row, which rows are generally horizontal and parallel to one another. On the other hand, in inverted groups with single-wire draw, the reversing cylinders are placed in the upper row and the drying cylinders in the lower row. In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers, of the type mentioned above.
When a paper web is dried by means of normal groups with single-wire draw from the side of only its bottom face, it is dried asymmetrically and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, seen from above.
As known in the prior art, the tendency of curling of paper is already affected in connection with the web formation, in particular at the sheet formation stage by means of the selection of the difference in speed between the slice jet and the wire, and by means of other running parameters during production of the paper web. As known from the prior art, for example, in the case of copying paper, by means of unequalsidedness of drying in the after-dryer, a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying may be optimized. In the case of copying paper, the reactivity of curling, i.e., the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper web, which is produced in connection with the web formation in the wet end of the paper machine.
The most recent technology related to the present invention in high-speed paper machines, in particular in fine-paper machines, has been based on dryer sections in which there is single-wire draw over the major part of the length of the machine and, in view of controlling the tendency of curling of paper, in practice, an inverted group has also been used in order that the drying can be made sufficiently symmetric in the z-direction (the direction of thickness of the web).
In conventional after-dryers in paper machines, in which the web is dried after coating or surface-sizing, a significant problem is presently the swelling of the web arising from the moisture introduced by the coating into the web, which swelling produces "wrinkles" and equivalent problems that deteriorate the runnability in the web. In twin-wire draw, which is traditionally used in an after-dryer, the web flutters between the upper and lower rows, in which connection, web breaks tend to occur and, moreover, on the drying cylinders in the lower row, when the web is wrinkled, the drying wire of the lower row has been readily damaged as a result of the wrinkles. This results in frequent replacements of the lower fabric.
Further, as known from the prior art, problems have also been caused by the lowering of the web strength resulting from the coating, in which connection, long free draws of the web have been quite problematic.
From the prior art, constructions are known for an after-dryer for paper to be coated, in particular for fine paper or equivalent, in which dryer there is first an upper cylinder and a lower cylinder and after this, one group that employs normal single-wire draw, and after the single normal-wire draw group, dryer groups that make use of twin-wire draw. In these applications, it is a problem that, in view of the tendency of curling of paper, the ratio of the upper and lower cylinders is incorrect if the curling is to be regulated efficiently. Further, in conventional after-dryers, it has been necessary to keep the temperature of the first cylinders low because of the adherence of the web and the size/paste to the cylinder.
Groups of the sort mentioned above for finishing of paper to be coated, in particular of fine paper, have been described, among other things, in the current assignee's Finnish Patent Application No. 950434 corresponding to the current assignee's U.S. patent application Ser. Nos. 08/467,780 (now abandoned) and 08/705,059 incorporated by reference herein.